Previous studies of this Collaborative Chemoprevention Project demonstrated potent inhibitory activities of phenethyl isothiocyanate (PEITC) and other isothiocyanates against nitrosamine-induced carcinogenesis. Program 3 studied the most likely mechanism of this action, namely the inhibition of the bioactivation of nitrosamines by isothiocyanates. This program plans to further elucidate the biochemical mechanisms of the inhibition with the following specific aims: 1. To identify and characterize the enzymes which are responsible for the oxidation of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) in mouse and human lungs. 2. To elucidate the mechanisms of inhibition of NNK metabolism by PEITC and other isothiocyanates. 3. To elucidate the mechanisms of the inhibition of N- nitrosomethylbenzylamine (NMBzA) metabolism by PEITC and to identify the major enzymes involved in the catalysis. 4. To delineate the actions of PEITC on microsomal enzymes by studying the suicide inactivation of P-450 2E1 by PEITC and the covalent binding of PEITC to microsomal proteins. 5. To fully characterize the effects of PEITC on different xenobiotic- metabolizing enzymes in the liver and other target organs in vivo and in vitro, and to determine the effects of PEITC on the metabolism of NNK, NMBzA, N-nitrosodimethylamine, and benzo(a)pyrene. 6. To elucidate the structure-activity relationship of the inhibitory actions of naturally occurring and synthetic isothiocyanates in the metabolism of selected carcinogens and to offer information on their possible activities in the inhibition of carcinogenesis. The project is expected to provide important information on the enzymology of NNK and NMBzA bioactivation as well as the mechanisms of inhibition of this process by isothiocyanates. The information is important for evaluating the potential usefulness of isothiocyanates and other compounds as chemopreventive agents against human cancers.